bug#36609: 27.0.50; Possible race-condition in threading implementation

[dick . r . chiang]

> You cannot have, for example, half of the bits of the variable set by one 
> thread
> and the other half by another thread.

I dunno, this seems like a sophomoric, rose-colored view of variable sharing
without regard to caching, or really any generally accepted guarantees about
POSIX threads.

> So can you describe what you think happens in your scenario that the
> offending change that added threads_holding_glib_lock causes?

Two threads see `threads_holding_glib_lock` is "0" and both attempt
acquiring the lock (line 43, xgselect.c)

Or, two threads see `threads_holding_glib_lock` as "2" and glib
lock is never released. (line 37, xgselect.c)